Experiments have been designed to elucidate the factors involved in cellular resistance to and recovery from virus infection. The major areas which will be investigated are: (1) persistent viral infections in cell culture model systems, with emphasis on the factors involved in establishment and maintenance of the persistently infected state and the analysis of evolution of virus populations during the course of persistence; especially the selection of mutants with temperature-sensitive or other phenotypes; (2) the factors involved in determining the sensitivity of different viruses to interferon; and (iii) the control mechanisms involved in double infection of cells with two viruses, using model systems in which the double infection results in either facilitation or inhibition of one of the infecting viruses.